The present invention relates generally to utility power conservation devices and more particularly to an electric utility demand limiting device and method for controllably connecting and disconnecting a utility customer from a utility power system.
As the cost and difficulty of providing additional electrical power generating capacity increases, electrical power utilities have sought ways to limit or decrease the peak electrical power demand that may occur throughout a typical operating day. By limiting or lowering this peak demand, additional generating capacity need not be added and the existing generating capacity may be more efficiently utilized.
Various devices, systems and methods have been proposed toward this end. For example, in order to vary the cost of electric power to a customer and to thus discourage power consumption when overall demand is high, U.S. Pat. No. 3,683,343 to Feldman, et al. describes a remotely controlled demand metering system. The meter, upon reception of suitably coded signals from a power station, records a number of kilowatt-hours greater than actually consumed. This metering system, however, only encourages voluntary conservation by the customer and thus does not effectively limit the customer's electrical power consumption to some preset limit.
As another incentive for reducing power consumption during peak demand periods, the utilities have instituted demand billing wherein the cost of electrical power varies with the customer's maximum demand during a predetermined time period. Various devices, systems and methods are known that can be used at the utility customer's site to monitor power consumption and to disconnect one or more loads to thus control the customer's peak power consumption. Such devices, systems and methods are described, for example, in U.S. Pat. Nos. 3,789,236, 4,075,699, 4,117,537, 4,125,782, 4,125,895, 4,153,936, 4,167,679 and 4,168,491. The patent to Lacroix, U.S. Pat. No. 3,789,236, also describes a device that can sound an alarm when power consumption reaches a preselected threshold.
These devices, however, do not respond to a remote control signal from a utility company. Thus, although there may actually be no necessity for limiting power demand, the devices will still function to disconnect loads or sound alarms, consequently resulting in greater customer inconvenience than is necessary. Also, these devices generally integrate power consumption with respect to time. If the customer's load includes devices or machines that have a high inrush current as occurs, for example, during the start of electric motors, this integration technique can cause the control device to disconnect loads prematurely.
Moreover, since these devices disconnect or connect specific loads in response to excessive power consumption, it is not possible for the customer to easily select the particular load that is desired to be disconnected at a particular time. This can result in inconvenience where, for example, the control device controls an air conditioning compressor but the customer instead desires to continue operation of the compressor and to instead disconnect some other load such as a swimming pool pump motor or an electric water heater. This leads not only to increased customer inconvenience but also can result in substantial customer dissatisfaction.
Further disadvantages of the control devices discussed in the above-referenced patents is that the devices are generally relatively complex and that it is often difficult to retrofit such a device at the customer's site because extensive wiring and modification of various of the customer's loads is required.
Yet other devices and systems are known which disconnect specific customer loads in response to a control signal generated at a remote point. Such devices and systems are generally described in U.S. Pat. Nos. 3,906,242, 4,023,043, 4,130,874, 4,135,181, 4,152,605 and 4,161,720. The systems can allow an electric utility to disconnect certain of the consumer's loads during peak power demand times so that the peak power demand can be reduced.
Such devices and systems, however, disconnect or shed selected customer loads regardless of the total power consumption by the individual customer. Although this can help to limit peak demand, some customers may still retain a relatively large power demand during this period. Also, as discussed above, these devices and systems do not allow an individual customer to select the particular load that the customer desires to do without during the peak demand period.
Also, several of the systems require relatively complex and expensive control centers and correspondingly complex and expensive devices at the customer's site. The individual customer loads to be disconnected must also be modified and connected to the control device. Although the control devices may disconnect specified loads, these control devices do not encourage overall utility customer conservation since the remaining customer load is unaffected. Furthermore, since only a portion of the customer's total load is controlled, it is more difficult for the utility company to accurately project the peak power demand that may occur at a given time.